The aim of the invention is low cost production process of metals and alloys, which are reactive at high temperature in the manufacturing cycle. Reactive alloys might be determined as alloys that exhibit an increase in chemical interaction with oxygen, nitrogen, carbon, etc. at elevated temperatures. Titanium aluminide, high strength titanium alloys, nickel aluminide, beryllium alloys, refractory metals, zirconium alloys, niobium, and other metals represent the group of such reactive alloys. Thin sheets or foils of reactive metals such as titanium aluminides are used for manufacturing important structural elements ideally designed for aircraft and space applications, where high service temperature and high strength-to-density components are required. However, the fabrication of such products as thin gage gamma-titanium aluminide sheet and foil is extremely difficult because of their inherent low ductility. In addition, oxidation of these alloys is drastically increased at elevated temperatures that significantly hinder hot forming of foil or thin sheet. Also, the undesired diffusion of a gas into a metal surface produces a decrease in ductility.
The aerospace industry continues to strive for larger production yields while reducing production costs, providing processing stability, and increasing the uniformity of microstructure of single-phase or multi-component titanium aluminide alloys.
The need for elevated temperatures during reactive metal processing has produced a number of previous techniques, which eliminate oxidation atmospheres from the environment of the metal during high-temperature processing. For example, hot working in large vacuum chambers or in inert gas environments is a common technique. However, the costly manufacturing facilities, which are required in these processes, add additional expenses to the final product. In many applications, an oxide layer is removed from a metal section by machining or the like.
Many technologies, known for manufacturing thin sections or foils of reactive metals, incorporate special coatings, claddings, or capsules that protect the reactive metal workpieces from oxidation and degradation during the hot forming process. For instance, in U.S. Pat. No. 3,164,884 to Noble et al., a method for the multiple hot rolling of sheets is disclosed, in which cover plates and sidebars are assembled around inner reactive metal plates separated by a release agent. The sidebars are welded to the cover plates and to each other along their outer edges. The release (separating) agents are water mixtures of aluminum, chromium, or magnesium oxides. Additionally built-in vent holes permit gases that are formed in the package to escape during the hot rolling process.
In U.S. Pat. No. 5,121,535 to Wittenauer et al., a method of forming a reactive metal workpiece was created, which is protected from high-temperature oxidation during hot working by placing the workpiece in a malleable metal enclosure with a film of release agents interposed between major mating surfaces of the reactive metal section and the metal jacket. In a preferred embodiment, a metal section of a reactive metal is placed in a non-reactive metal frame. The reactive metal section and frame are then interposed between non-reactive metals of the top and bottom plates, with a release agent which exhibits viscous glass-like properties at high temperatures being disposed at the interfaces of the reactive metal sections. The release agent is provided preferably in shallow depressions or pockets in the non-reactive sections where the metal interfaces. The assembly is then welded together near the perimeter so that the release agent is sealed in place between the sections.
The welded assembly may then be hot rolled under pressure to the desired gauge using conventional hot rolling machinery and procedures to form thin metal sections or foils. Other hot working techniques may be employed where suitable. As the assembly is hot rolled, the release agent flows to form a uniform interfacial film. Thus, accelerated oxidation during the high-temperature hot working of the reactive metal section is prevented using the present invention, by encapsulating the reactive metal section in a non-reactive metal jacket during hot working, with the major surfaces of the reactive metal core being separated from the encapsulant layers by a release agent.
Thereafter, the formed assembly or laminate is cooled, and the rolled assembly is sheared to remove the welded edges. The non-reactive metal sections are simply peeled from the reactive metal core by virtue of the presence of the brittle, non-cohesive release agent. Residual release agents can be removed from the finished reactive metal foil by a rinse. In this manner, U.S. Pat. No. 5,121,535 provides a method by which bulk quantities of reactive metals, such as refractory metals, can be formed into thin metal sections such as foils or strips without the use of vacuum processing equipment and with the utilization of conventional hot working equipment such as hot rolling machinery.
W. J. Truckner and J. F. Edd (U.S. Pat. No. 5,405,571) proposed a combination of tape casting and consolidation by hot pressing to manufacture thin sections from powders of titanium alloys, titanium aluminides, nickel aluminides, and molybdenum disilicide. The main drawback of this method is residual porosity that is present in the final alloy due to of traces of the polymer binder used in tape casting.
The U.S. Pat. No. 5,863,398 provides the manufacture of reactive alloys by hot pressing followed with sintering under pressure of 3000-5000 psi at 1300-1500° C. The method is characterized by low productivity and density gradient along the resulting thin material. This density gradient is caused by an error in parallelism between the punch and matrix of the hot pressing die that always exists in the procedure.
K. Shibue with coworkers reported on the manufacture of shaped TiAl alloy by cold extrusion of an elemental powder blend in an aluminum can followed by hot isostatic pressing (U.S. Pat. No. 5,372,663). This method can only be used only to produce symmetric articles, e.g., rod-like, but it is not suitable for thin sheets or strips.
U.S. Pat. No. 6,240,720 to Tseng, et al. discloses a multi-layer hybrid composite material formed as an elongated hollow cylindrical shell. The shell has a structure of inner and outer rims made from titanium alloy and a reinforcing layer of silicon carbide fibers between titanium rims. Mechanical properties and the reliability of such type of composites depend on the completion of the interface reaction between metal and ceramic fibers. Unfortunately, this reaction and the metal-ceramic bond strength are not controllable by parameters of the composite processing.
U.S. Pat. No. 4,816,347 to Rosental, et al. discloses a manufacture of hybrid titanium matrix composite comprising layers of titanium alloy and titanium aluminide reinforced by silicon carbide fibers. The composite is fabricated by CVD or PVD deposition of titanium aluminide layer on silicon carbide filaments, and layering these with Ti-6Al-4V alloy foils followed by HIP consolidation at 1700-1800 F. The vapor deposition involved in the manufacture constrains a thickness of the aluminide layer by microns, and significantly increases production costs. The contribution of thin aluminide films regarding strength of the composite structure is insignificant.
All previous technologies of fabricating thin sheets or foils from reactive alloys have considerable drawbacks, which make them undesirable in terms of sufficient protection from oxidation, cost, and production capacity, especially if the thin sections were produced initially from reactive alloy powders, which require additional hot working cycles for compacting. The resulting porosity causes very rapid oxidation of the reactive alloy to a substantial depth, and capsules designed in known inventions do not protect the sintered section from rapid oxidation. A significant difference in structures and mechanical properties between sintered sections, produced from reactive powder metal, and the frame (capsule), produced from non-reactive wrought metal, result in non-uniform deformation and stress concentration of the laminate package during the hot rolling process. Cracks occur in various areas of the sintered section during the first cycles of hot rolling and do not allow it to maintain a stable manufacturing process.
Therefore, it would be desirable to provide a cost-effective method of producing thin metal sections from powder reactive alloys, which reduces or eliminates destructive oxidation during high-temperature processing. The present invention achieves this goal by providing a method by which the powder of reactive metals can be formed into fully-dense thin sheets in a hot working process combining loose sintering, hot axial pressing, hot isostatic pressing, and/or longitudinal hot deformation followed by specified heat treatment.